1. Field of the Invention
The present invention relates to a Trench MOS Barrier Schottky Diode having an integrated Substrate-PN diode as a clamping element, hereinafter referred to as a TMBS-sub-PN, suitable for use as a Zener power diode having a breakdown voltage of approximately 20 V for use in vehicle generator systems.
2. Description of Related Art
In modern motor vehicles, functions are increasingly realized using electronic components. This creates a growing need for electrical power. In order to meet this need, the efficiency of the vehicle generator system has to be increased. Standardly, as a rule PN diodes are used as Zener diodes in the vehicle generator system. Advantages of the PN diodes are on the one hand their low blocking voltage and on the other hand high robustness. The main disadvantage is the high forward voltage UF. At room temperature, current does not begin to flow until UF=0.7V. Under normal operating conditions, e.g. a current density of 500 A/cm2, UF increases to greater than 1V, resulting in a non-negligible loss of efficiency.
Theoretically, a Schottky diode could be used as an alternative. Schottky diodes have a significantly lower forward voltage than do PN diodes, e.g. 0.5V to 0.6V at a high current density of 500 A/cm2. Moreover, Schottky diodes, as majority carrier components, have advantages during rapid switching operation. However, up to now Schottky diodes have not been used in vehicle generator systems. This is due to some decisive disadvantages of Schottky diodes: 1) higher blocking current in comparison with PN diodes, 2) stronger dependence of the blocking current on the blocking voltage, and 3) poor robustness, in particular during high-temperature operation.
Proposals have been made for improving Schottky diodes. One of these is the TMBS (Trench MOS Barrier Schottky diode), described for example in EP 0707 744 B1 or in published German patent document DE 694 28 996 T2. As FIG. 1 shows, the TMBS is made up of an n+ substrate 1, an n-epitaxial layer 2, at least two trenches 6 that are realized in n-epitaxial layer 2 by etching, metallic layers on the front side of chip 4 as an anode electrode and on the rear side of chip 5 as a cathode electrode, and oxide layers 7 between trenches 6 and metallic layer 4. Regarded electrically, the TMBS is a combination of an MOS structure (metallic layer 4, oxide layers 7, and n-epitaxial layer 2) and a Schottky diode (Schottky barrier between metallic layer 4 as anode and n-epitaxial layer 2 as cathode).
In the forward direction, currents flow through the mesa region between trenches 6. Trenches 6 themselves are not available for the flow of current. The effective surface for the flow of current in the forward direction is therefore smaller in a TMBS than in a conventional planar Schottky diode. The advantage of a TMBS lies in the reduction of the blocking currents. In the blocking direction, space charge zones form both in the MOS structure and in the Schottky diode. The space charge zones expand as the voltage increases, and, given a voltage that is less than the breakdown voltage of the TMBS, meet one another in the center of the region between adjacent trenches 6. In this way, the Schottky effects responsible for high blocking currents are shielded and the blocking currents are reduced. This shielding effect is strongly dependent on structural parameters Dt (depth of the trench), Wm (distance between the trenches), Wt (width of the trench), and To (thickness of the oxide layer); see FIG. 1.
A known procedure for producing the TMBS is as follows: realization of trenches 6 through etching of n-epitaxial layer 2; growth of oxide layer 7 and filling of the trenches with metal. The expansion of the space charge zones in the mesa region between trenches 6 is quasi-one-dimensional as long as the trench depth Dt is significantly greater than the distance between trenches Wm.
However, a decisive disadvantage of the TMBS lies in the weakness of the MOS structure. Upon breakdown, very high electrical fields arise within oxide layer 7 and directly in the vicinity of the oxide layer in n-epitaxial layer 2. The blocking currents flow mainly through the quasi-inversion layer of the MOS structure, along the trench surface. As a result, the MOS structure can be degraded through the injection of “hot” charge carriers from n-epitaxial layer 2 into oxide layer 7, and under certain operating conditions may even be destroyed. Because a certain amount of time is required for the formation of the inversion channel (deep depletion), the space charge zone can briefly expand further at the beginning of rapid switching processes, causing the electrical field strength to increase. This can result in a brief undesirable period of operation in breakdown. It is therefore not recommended to use TMBS as Zener diodes and to operate them in the breakdown region.
An alternative for improving the robustness of TMBS in breakdown operation is indicated by the TMBS-PN proposed in DE 10 2004 053 760. As FIG. 2 shows, the TMBS-PN is made up of an n+ substrate 1, an n-epitaxial layer 2, at least two trenches 6 etched into n-epitaxial layer 2, metallic layers on the front side of chip 4 as an anode electrode and on the rear side of chip 5 as a cathode electrode, and oxide layers 7 between trenches 6 and metallic layer 4. The lower area of trenches 8 is filled with p-doped Si or poly-Si. Metallic layer 4 in particular can also be made up of two different metallic layers situated one over the other, or of a combination of polysilicon and metal. For clarity, this is not shown in FIG. 2.
Regarded electrically, the TMBS-PN is a combination of an MOS structure (metallic layer 4, oxide layers 7, and n-epitaxial layer 2), a Schottky diode (Schottky barrier between metallic layer 4 as anode and n-epitaxial layer 2 as cathode), and a PN diode (PN transition between p-tubs 8 as anode and n-epitaxial layer 2 as cathode). In the TMBS-PN, as in the conventional TMBS, currents in the forward direction flow only through the Schottky diode if the forward voltage of Schottky diode 4, 2 is significantly smaller than the forward voltage of the PN diode.
In the blocking direction, space charge zones form in the MOS structure, the Schottky diode, and the PN diode. The space charge zones expand as the voltage increases, and, given a voltage that is less than the breakdown voltage of the TMBS-PN, meet one another in the center of the region between adjacent trenches 6. In this way, the Schottky effects responsible for high blocking currents are shielded and the blocking currents are reduced. This shielding effect is strongly dependent on structural parameters Dox (depth of the trench portion having the oxide layer), Wm (distance between the trenches), Wt (width of the trench or of the p-tub), Dp. (depth of the trench portion having p-doped Si or poly-Si=thickness of the p-tub), and To (thickness of the oxide layer); see FIG. 2.
The TMBS-PN has a similar shielding effect on Schottky effects as does a TMBS, but in addition also offers high robustness due to the clamping function. The breakdown voltage of PN diode BV_pn is designed such that BV_pn is lower than the breakdown voltage of Schottky diode BV_schottky and the breakdown voltage of the MOS structure BV_mos, and the breakdown takes place on the floor of the trenches. During breakdown operation, blocking currents then flow only through the PN transitions, and not through the inversion layer of the MOS structure as in a TMBS. The TMBS-PN thus has robustness similar to a PN diode. Moreover, with the use of the TMBS-PN there is no need to fear the injection of “hot” charge carriers, because the high field strength during breakdown is not situated in the vicinity of the MOS structure. As a result, the TMBS-PN is well-suited for use in vehicle generator systems as a Zener power diode.